Activated alumina process, regenerant

I. Activated Alumina Process with Regenerant Reuse... 

For regeneration to be technically viable, it must be able to remove deposited vanadium and nickel quantitatively as well as the carbonaceous coke which was co-deposited. The catalyti-cally active metals should remain unaffected in amount, chemistry, and state of dispersion. The alumina support should remain intact, with the surface area, pore-size distribution and crush strength after treatment comparable to that of the original. To be economically viable, the process should be accomplished in a minimum of steps at nearly ambient temperatures and preferably in aqueous solution. The ultimate proof of any such scheme is for the catalytic activity of the regenerated catalyst to be equal to that of a fresh one. 

In this process, propane, and a small amount of hydrogen to control coking, are fed to either a fixed bed or moving bed reactor at 950—1300° F and near atmospheric pressure. Once again the catalyst, this time platinum on activated alumina impregnated with 20% chromium, promotes the reaction. In either design, the catalyst has to be regenerated continuously to maintain its activity. 

Zeolites have also proven applicable for removal of nitrogen oxides (NO ) from wet nitric acid plant tail gas (59) by the UOP PURASIV N process (54). The removal of NO from flue gases can also be accomplished by adsorption. The Unitaka process utilizes activated carbon with a catalyst for reaction of NO, with ammonia, and activated carbon has been used to convert NO to N02, which is removed by scrubbing (58). Mercury is another pollutant that can be removed and recovered by TSA. Activated carbon impregnated with elemental sulfur is effective for removing Hg vapor from air and other gas streams the Hg can be recovered by ex situ thermal oxidation in a retort (60). The UOP PURASIV Hg process recovers Hg from clilor-alkali plant vent streams using more conventional TSA regeneration (54). Mordenite and clinoptilolite zeolites are used to remove HQ from Q2, clilorinated hydrocarbons, and reformer catalyst gas streams (61). Activated aluminas are also used for such applications, and for the adsorption of fluorine and boron—fluorine compounds from alkylation (qv) processes (50). 

Catalyst decomposition depends heavily on the specific process conditions employed. Producers operate under two different regimes the all-tetra system, in which no specific actions are taken to either suppress the formation of tetra or to dehydrogenate it back to anthraquinone, and the anthra system, in which efforts are made to minimize the tetra content. Tetra formation can be reduced by the use of selective hydrogenation catalysts and specific operating conditions (i.e., solvent choice and specialized quinones). In addition, tetra can be dehydrogenated in the presence of activated alumina (AI2O3) in the catalyst regenerator (Eq. (14.12)). 

The atmosphere can be refined further by reducing the water-vapour content. This can be accomplished by refrigeration causing the water to condense. The temperature of this treatment is normally restricted to 5 °C. Otherwise ice may form and block the process whereas water can easily be drained away. More effective drying down to dew points in the region of -40 °C can be achieved using activated alumina or silica-gel towers. Used in pairs, normally one absorber is operational while the other is regenerated. 

For commercial applications, an adsorbent must be chosen carefully to give the required selectivity, capacity, stability, strength, and regenerability. The most commonly used adsorbents are activated carbon, molecular-sieve carbon, molecular-sieve zeolites, silica gel, and activated alumina. Of particular importance in the selection process is the adsorption isotherm for competing solutes when using a particular adsorbent. Most adsorption operations are conducted in a semicontinuous cyclic mode that includes a regeneration step. Batch slurry systems are favored for small-scale separations, whereas fixed-bed operations are preferred for large-scale separations. Quite elaborate cycles have been developed for the latter. 

Usually, under heating the destruction of organic compounds to be evaporated occurs on the surface of active alumina sorbents. This phenomenon results in the formation of harmful volatile compounds and the coke that decreases adsorbent capacity. Therefore, it seems more preferable to oxidize organic compounds to carbon dioxide and water during adsorbent regeneration. The processes and purification systems that combine the adsorption and the catalytic combustion regeneration were developed. The transition aluminas doped with catalytic... 

The S Zorb and IRVAD processes are based on adsorbing the more polar AAT compounds on solid material such as treated alumina or silica. Continuous removal of the saturated adsorbent permits recovery of the AAT compound and recycle of the activated adsorbent. Processing cost results primarily from the consumption of fuel, makeup adsorbent and capital charges. The cost of adsorbent regeneration may represent a limitation for the applicability of this type EDS process, and other constraints may be limitations to the feedstock boiling range and sulfur content. 

Conventional defluoridation systems employed in India such as adsorption using activated alumina or Nalgonda technique, which involves chemical precipitation with lime-alum mixture cannot remove all the impurities in a single step like RO does. RO can be scale-up to higher community-based capacities, which is not feasible in case of the aforementioned methods. Moreover, RO can also remove microbial content, which the conventional methods cannot. The price of water treated by RO is competent with other methods and more economical than ion-exchange process, which requires cumbersome regeneration procedures. 

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